1. Field of the Invention
The invention relates to recovery of heavy oil. In one aspect, the invention relates to establishment of a heated permeability zone in an unconsolidated heavy oil sand reservoir suitable for recovery of the heavy oil by heated fluid displacement. The invention has particular utility in recovery of heavy oils or tars from Athabasca class deposits, although it is also useful for recovery of oils having higher API gravity, particularly from relatively thin reservoirs or shallow reservoirs having relatively low permeability.
2. Brief Description of the Prior Art
The following comprises a prior art statement in accord with the guidance and requirements of 37 CFR 1.5, 1.97, and 1.98.
There are many subterranean heavy oil containing formations throughout the world from which the oil cannot be recovered by conventional means because of its high viscosity. The so-called tar sands or bitumen sand deposits are an extreme example of such viscous heavy oil containing formations. A tremendously large energy resource [some 2,100-billion barrels (334.times.10.sup.9 m.sup.3)--almost as much as the world's known reserves of lighter oil] is available in such deposits, provided that technology is developed to recover such heavy oil at favorable economics.
One of the larger of the tar sands deposits is located in the northeastern part of the province of Alberta, Canada, and is estimated to contain in excess of 700 billion barrels (112.times.10.sup.9 m.sup.3) of heavy oil. The tar belts of Venezuela are reputed to contain even larger quantities than the rest of the world combined. Lesser deposits are located in Europe and Asia. In the U.S., extensive tar sands deposits exist in California, Utah, Texas, and elsewhere. One resource of particular interest includes tar sand deposits in Maverick and Zavala Counties, of South Texas, which are estimated to contain 10 billion barrels (1.60.times.10.sup.9 m.sup.3) or more of very heavy oil or tar having an API gravity in the range of -2 to +2. In certain aspects, the Texas tar sands are even more difficult to recover heavy oil from than the Athabasca tar sands. The tar in this resource is essentially a solid at reservoir temperature. An exemplary San Miguel sand of the resource averages about 50 feet (15.24 m) in thickness with a permeability of about 500 to 1000 millidarcies [0.49(.mu.m).sup.2 -0.99(.mu.m).sup.2 ] and about 30 percent porosity. Initial oil saturation is about 55 percent and depth is about 1500 feet (457 m).
Such heavy oil deposits or tar sands deposits of the Athabasca class or type, in which the invention is most useful, can be generally described with reference to the Athabasca deposits as an example. The Athabasca heavy oil or tar sands are described as sand saturated with a highly viscous heavy crude oil not recoverable in its natural state through a well by ordinary petroleum recovery methods. The oil is highly bituminous in character with viscosities up to millions of centipoise at formation temperature and pressure. The API gravity of the heavy oil ranges from about 10.degree. to about 6.degree. in the Athabasca region and on down to negative numbers in other deposits such as the Maverick County, Texas, deposits. At higher temperatures, such as temperatures of above about 200.degree. F. (93.degree. C.), this heavy oil becomes mobile, but at such temperatures the heavy oil deposits are incompetent or unconsolidated. The oil content of the deposit generally is about 10 to 12 percent by weight, although sands with lesser or greater amounts of oil content are not unusual. Additionally, the sands generally contain small amounts of water, generally about 3 to about 10 percent by weight. The deposits are about 35 percent pore space by volume or 83 percent sand by weight. The sand is generally a fine-grain quartz material. One of the striking differences between such deposits and more conventional petroleum reservoirs is the absence of a consolidated matrix. While the sand grains are in grain-to-grain contact, they are not cemented together.
Excellent descriptive matter relating to tar sands is found in "The Oil Sands of Canada--Venezuela, 1977", CIM Special Volume 17, The Canadian Institute of Mining and Metallurgy (1977), which represents the collective proceedings of the Canada-Venezuela Symposium, held in Edmonton, Alberta, Canada, May 30th to June 4th, 1977.
In contrast to the situation relating to the Athabasca class deposits, a variety of processes are available to the industry for the recovery of heavy oil from many consolidated reservoirs having appreciable fluid permeability, provided that such reservoirs are thick enough for economic recovery.
For example, forward combustion and water modified forward combustion or fire flooding processes are being successfully employed in a number of such reservoirs. Detailed information relating to a project involving such processes is available by way of the U.S. Department of Energy under Contract EY-76-C-03-1189, wherein Cities Service Company as contractor is conducting improved oil recovery by in situ combustion in the Bellevue Field in Louisiana. Considerable other information on such processes is published and available from a number of sources.
A second type of thermal recovery processes include the steam and hot water injection processes.
With the so-called huff-and-puff process, steam is injected into a producing well, the well is allowed to soak for a while, and then fluids including mobilized oil are produced. A variety of successful huff-and-puff projects are in operation and considerable data are published.
Essentially two separate types of hot water and steam injection processes involving fluid displacement are in use.
The first type is a drive or matrix flow process in which hot water or steam, or some intermediate mixture is continuously injected into a reservoir at relatively low rates and pressures to heat and displace oil in a modified water flooding manner. This technique works satisfactorily if the oil at natural reservoir conditions is sufficiently mobile to be moved at practical rates by hot fluid injection without vertical parting of the reservoir or uncontrolled viscous fingering and tonguing. Earlier successful uses of this process have been employed at Kern River, California, the Schoonebeek Field in the Netherlands, and Tia Juana Field in Venezuela. Many more recent successful uses of this method have also been employed.
A second type of displacement process, which can be referred to as a conduction heating steam flood, involves conduction heating of a reservoir from hot fluid passing through a highly permeable zone, such as a horizontal fracture, a gas cap at the top of the reservoir, or a relatively thin section of permeability within or adjacent to the main pay zone such as a water zone at the bottom of the deposit. The reservoir section adjacent to the highly permeable zone is heated by vertical conduction of heat from steam or hot water in the channel and also by condensation of steam or transfer by hot water which may have leaked from the channel. If a permeability channel can be opened and kept opened until flow of heated heavy oil is established, this type process has application to heavy oil reservoirs in which the reservoir fluids are essentially immobile at reservoir temperature.
In SPE Paper No. 1950 by Abdus Satter (prepared for the 42nd Annual Fall Meeting of the Society of Petroleum Engineers of AIME held in Houston, Texas, Oct. 1-4, 1967); Doscher et al (Petroleum Engineer, January (1964) pp. 71-78) are cited as reporting that Shell Oil Company carried out the first known conduction heating operation in the Athabasca tar sands. Therein it is reported that a horizontal fracture was propagated between the injection and production wells in the Athabasca sand followed by steam and aqueous solution injection to produce at least some oil-in-water emulsions to demonstrate the theoretical viability of the approach.
Since then, a number of approaches involving fracturing followed by conduction heating steam flooding have been proposed.
An unsuccessful attempt to unlock Utah tar sands is reported by Thurber, Petroleum Engineer, November (1977) pp. 31-42.
However, it has been and is recognized in the art that conduction heating steam flooding requires the establishment of a communication path between an injection and a production well through which the fluids may be passed. As is pointed out by Doscher et al in U.S. Pat. No. 3,221,813 (which may disclose the closest approach to our invention), conventional thermal drive processes do not generally prove effective in recovering oils from heavy oil deposits of the Athabasca type. Such heavy oil sands at the natural temperatures of the deposits are not sufficiently permeable to allow the steam or other hot fluids to pass through the deposits to effectively lower the viscosity of the oil therein. Neither has use of conventional sand packed fracturing proved sufficient to make thermal drives in Athabasca class heavy oil deposits practical. Such fractures tend to close as soon as the pressure utilized to create them is relieved. Upon this occurrence, the unheated tar sand reverts to its impermeable state and is not subject to production with conventional thermal drive processes. In a competent formation, the closing of such a fracture can be avoided by introducing propping agents such as granular materials into the fracture to hold it open. This method, however, is ineffective in respect to an incompetent heavy oil-bearing formation such as an Athabasca type tar sand. Such tar sands are relatively soft and subject to plastic flow. Thus, even if a sand packed fracture is produced, as soon as the walls of the fracture become heated, the incompetent formation slumps between the grains of the propping agent and permeability is lost. Also, any bitumen heated by the injected fluid will flow in an unheated or less than adequately heated fracture zone for only a brief period before it loses heat and becomes so viscous that it is essentially immobile, resulting in the plugging of the channel. Such problems relating to establishing and maintaining fluid mobility between the injection and production wells, particularly near the production well, are also of critical importance with lighter heavy oils, particularly those that have considerable viscosity at or near reservoir temperature.
In addition to the approach involving attempted formation of aqueous emulsions with aqueous caustic solutions as proposed in Petroleum Engineer, January (1964) pp. 71-78, various other processes have been proposed as are disclosed in the following references: U.S. Pat. Nos. 4,068,716; 3,881,551; 3,342,258; 2,876,838; 2,813,583; 4,068,717; 3,613,785; 3,346,048; 3,810,510.
The closest approach of the prior art to their invention with which the inventors are familiar is exemplified by the following five patents. The problem of viscous tar plugging of the communication channels between the wells in Athabasca type heavy oil sands at the cooler downstream end of the channels is recognized by this prior art and proposals are made to deal with it in a number of ways which are different from the process of the invention.
Three patents assigned to Shell Oil Company, namely U.S. Pat. Nos. 3,221,813, 3,379,250, and 3,396,791, appear to be the most relevant. U.S. Pat. No. 3,221,813 discloses fracturing between an injection and a production well in a tar sand formation, injecting steam at floating pressures into the injection well, and periodically removing viscous tar plugs in the channel by circulating a tar entraining liquid such as a petroleum emulsifier or a petroleum solvent. U.S. Pat. No. 3,379,250 discloses a process wherein a hydraulic fracture is established between a production well and an injection well in a tar sand formation and a heated channel is formed therebetween by circulating water through the fracture while raising its temperature gradually such that no more than 1.degree. F. temperature differential per foot occurs. U.S. Pat. No. 3,396,791 discloses a process wherein a hydraulic fracture is established between an injection well and a production well in a tar sand formation, water of increasing temperatures is circulated through the fracture until the viscosity of the tar is less than about 50 cp. and then steam is passed through the formation from the injection well to the production well.
U.S. Pat. No. 3,908,762 discloses establishing a hydraulic fracture between an injection well and a production well traversing a tar sand formation, and then establishing a heated permeability zone between the wells by injecting steam plus a noncondensable gas at a pressure not exceeding a value in psi numerically equal to the overburden thickness in feet. Including the noncondensable gas (such as CO.sub.2, methane, nitrogen, or air) along with the steam injected is purported to alleviate the problem of viscous tar plugging the channel at the cooler production well end during the steam injection step.
U.S. Pat. No. 3,411,571 discloses horizontally fracturing and propping between a production well and an injection well traversing a tar sand formation, passing steam from the injection well to the production well, then steam from the production well to the injection well, and then fire flooding from the injection to the production well. The process disclosed therein does not appear to address the problem of plugging of the fracture when tar mobilized by the steam flood flows into cooler regions, and does not appear to be suitable for very heavy Athabasca type heavy oil sand deposits.
Though the processes disclosed by the prior art have considerable merit, and in fact are quite useful for recovering heavy oil from reservoirs which are consolidated and wherein the heavy oil is substantially less viscous than Athabasca type heavy oil, commercially successful recovery of heavy oil from an Athabasca type deposit, that is wherein the heavy oil is very viscous at reservoir temperature and less than 10 API gravity, wherein the reservoir is incompetent, and wherein the reservoir is substantially impermeable at its natural temperature, has not yet been demonstrated. The closest approach to commercially recovering heavy oil from such reservoirs involves a special case wherein a water zone through which fluid communication may be established lies adjacent to and below the heavy oil deposit. The process disclosed and claimed herein provides a breakthrough for commercial oil recovery from such deposits.
The processes of the prior art are also less than adequate for economic recovery of higher API gravity heavy oil from deposits which are relatively thin, of shallow depth, or of low permeability, particularly with an economically feasible distance between wells. In such reservoirs, an uneconomically large amount of steam is wasted by the prior methods in heating underburden and overburden in order to heat and recover a given amount of heavy oil.
Even in reservoirs subject to feasible recovery by thermal processes presently available, considerable improvement is needed in thermal efficiency. Thermal efficiency "TE.sub.RH " is discussed by P. E. Baker, "Heat Wave Propagation and Losses in Thermal Oil Recovery Processes", Proceeding of the 7th World Petroleum Congress--1967, Volume 3, p. 459-70. This publication, which is herewith incorporated by reference, defines TE.sub.RH (Thermal efficiency for reservoir heating) by: ##EQU1## wherein K.sub.ob is the thermal conductivity of the overburden, a determined value normally expressed in Btu/hr-ft-.degree.F. (or alternate metric terms);
wherein erfc is the complimentary error function obtainable from standard math tables of tabulated values; PA1 wherein h is the measured value of thickness of the heated reservoir body, normally expressed in feet (or alternate metric terms); PA1 wherein .rho.C is the measured heat capacity of the material in point, normally expressed in Btu/ft.sup.3 -.degree.F. (or alternate metric terms); PA1 wherein .rho. is the determined (measured) density of the material, normally expressed in lbs/ft.sup.3 (or alternate metric terms); PA1 wherein C is the determined specific heat capacity of the material, normally expressed in Btu/lb.-.degree.F. (or alternate metric terms); PA1 wherein (.rho.C).sub.ob is the heat capacity of the overburden; PA1 wherein (.rho.C).sub.r is the heat capacity of the reservoir; and PA1 wherein t is time, usually expressed in days or hours. PA1 (a) hydraulically fracturing between the wells, PA1 (b) injecting hot aqueous fluid into the injection well, and PA1 (c) producing fluids from the production well; PA1 (a) hydraulically fracturing between the wells PA1 (b) injecting steam into the injection well, and PA1 (c) producing fluids from the production well; PA1 (a) penetrating the tar sand formation with an injection wellbore and a production wellbore horizontally separated from each other; PA1 (b) hydraulically and/or explosively fracturing from the production well; PA1 (c) injecting steam into the production well to part the fracture zone and impart heat to it; PA1 (d) hydraulically fracturing from the injection well; PA1 (e) injecting hot water and/or steam into the injection well at a sufficient rate and pressure to part the formation along the fracture system between the wells and thus form a heated channel of mobilizable tar in the formation in proximity to the fracture system between the wells; and PA1 (f) passing hot water and/or steam into the injection well and fluids through the heated permeable channel between the wells to effect conduction heating steam flooding therebetween with tar recovery from the production well.
In essence, TE.sub.RH (thermal efficiency for reservoir heating) is the fraction of heat at a point in time that is imparted into and is maintained in the reservoir relative to the total heat injected. Typically, TE.sub.RH ranges from about 20 to 40 percent 0.2 to 0.4 for prior art processes. None are known of having a value for TE.sub.RH over about 40 percent. During the high rate injection of our process, TE.sub.RH is over 40 percent, typically is in the range of 70 to 90 percent, and may approach 100 percent.
As is well known to those skilled in this art, thermal efficiency is a key to economics and economics is the key to feasibility. One simply cannot spend more on energy or otherwise to recover heavy oil than the heavy oil is worth.